Study on the Influence of Floating Ring Axial Length on Vibration Characteristics of High-speed and Light-load Turbocharger Rotor System

The floating ring bearing axial length has an important effect on the distribution of oil film pressure and the eccentricity ratios, which leads to obvious sub-frequency vibration and nonlinear failure for the high-speed and light-loaded turbocharger rotor. Based on fluid lubrication theory and balance equation of floating ring moment, the dynamic equation of turbocharger rotor system with floating ring bearing is derived, and the relationship between floating ring bearing axial length and the vibration response of rotor system is revealed. Taking the turbocharger rotor system for a gasoline engine as an example, the influence of inner and outer axial lengths on the dynamic characteristics of the bearing oil film pressure and eccentricity ratios is analyzed, and the dynamic finite element model of the rotor system is constructed, the transient vibration response in frequency domain of rotor system with different floating ring axial lengths is studied by means of three-dimensional vibration waterfall. It shows that the floating ring inner axial length is increased from 2.6 to 4.6 mm, the floating ring rotation speed is increased, the maximum internal oil film pressure and the journal eccentricity ratios are decreased, the frequency division amplitude is increased, and the shaft speed of emerging sub-frequency is decreased from 142 kr/min to 76 kr/min. This also means that it is easier to generate obvious nonlinear whirling. When outer axial length increased from 3.6 to 6.15 mm, the floating ring speed and maximum outer oil film pressure are decreased respectively, the eccentricity ratios of the floating ring and the shaft neck relative to the floating ring are decreased, the sub-frequency amplitude is decreased at low speed, and the shaft speed of emerging frequency division is increased from 10 kr/min to 22 kr/min, which can restrain the premature nonlinear whirl of the rotor system. The results can provide theoretical support for structural parameters design and experiment of floating ring bearing.